Impact on Vietnam

Water resources

Global warming is likely to affect the availability and quality of water resources throughout Vietnam as temperature rise affects evaporation rates and rainfall patterns alter.

Climate model projections have been used to assess likely impacts on water resources in North, Central and South Vietnam up to the year 2030.

Most water resources in the North come from the Red River and the Thai Binh river system, with an annual flow of 137,000 million m³ of which 90,000 million m³ are generated in the territory of Vietnam.

With the two existing Hoabinh and Thacba reservoirs, the 250-year flood (as experienced in 1971) can be kept under the critical level at Hanoi which is 13.3 m above mean sea level. However, this level still threatens the dyke systems in the delta.

The construction of more cascades on the Da and Lo Rivers would provide more flood control and more water for hydroelectricity and irrigation. But the increase in annual mean runoff and flood seasonality and the decrease in vulnerability to drought in North Vietnam predicted by climate models suggests that there are some problems still to be solved.

First, the reservoir operation rules need to be reviewed and made more flexible. It is necessary to control flooding in high areas of the flood plain at the same time as avoiding the risk of filling the reservoirs too soon in the summer or not completely filling the reservoirs in the autumn.

Second, coastal management will have to be considered. With the present sedimentation rate, the shoreline is expanding at 50-100 m per year in some places. But as a result of sea level rise and the trapping of sediment in the reservoirs, the shoreline may retreat in the future. Land presently at the margins of tide and wave action may be inundated and accelerated erosion of dunes may occur in response to higher wave action associated with raised water levels.

Finally, saline intrusion in the lower reaches of the deltas would be aggravated by the predicted sea level rise and may warrant revised management of flow control.

In the north of Central Vietnam, an increase in annual runoff is predicted. This is associated with more intense typhoon activity and flood severity.

In contrast, in the middle and southern region of Central Vietnam, annual runoff may decrease, particularly in the Thuan Hai province. Vulnerability to drought would increase and desertification would become the foremost problem.

Most of the land in the south of Vietnam is drained by the Mekong River and the Dong Nai River. The Lower Mekong Basin covers an area of 620,000 km² and yields an annual flow of 475,000 million m³.

The marked decrease in annual runoff predicted for this region would enhance the vulnerability to drought in the delta areas unless large reservoirs are built upstream.

Within Vietnam, the Cuu Long Delta covers an area of 64,300 km² of which 2.4 million hectares are cultivated area, and is inhabited by a population of some 17 million (37% of the total population in the basin). Its annual flow is only 50,000 million m³. The Cuu Long Delta suffers from drought almost every year (January-May) and is damaged by flood every seven to twelve years. Floods occurred in 1966, 1978, 1984 and 1991.

From the extreme events of the past, the experience has been gained to cope with short-term variability, by changing existing reservoir operating rules and improving flood and drought preparedness.

But as far as long-term variations and changes in water resources are concerned, more reliable assessments are needed from which water resource managers and planners will elaborate good planning for the future.

The potential change in the demand side of water resource management may also be significant. For example, in regions in which rainfall becomes less abundant and where temperatures rise, there may well be an additional demand for irrigation, water supply and cooling techniques in industry. Such considerations must play an important role in long-term planning.

It will also be necessary to consider the wider situation. Measures taken in response to changing water availability higher upstream in neighbouring nations may have serious consequences for riverflow within Vietnam.

Based on material provided by
Mr Dao Van Le, Hydrometeorological Service

"Impact on Vietnam: The mangrove ecosystem"

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